Effort has been devoted in Silicon (Si) compatible photonics during the last two decades, especially in the field of active devices. Active building blocks have been fabricated on Si optical modulators and Germanium (Ge) photodetectors, with some showing integration with standard complementary metal-oxide-semiconductor (CMOS) circuits or other optical components.
Ge has been investigated for active photonics devices, particularly for light detection at optical communication wavelengths (of around 1.3 to 1.55 μm). High performance Ge photodetectors have been realized due to its favorable absorption coefficient at these wavelengths.
A Ge photodetector needs low dark current, high responsivity and high bandwidth for optimum performance. Doping profiles must be optimized such that the electron-hole generation region remains intrinsic during operation. Phosphorus diffusion in Ge is extremely fast, resulting in non-abrupt junctions and higher dopant concentrations in intrinsic regions. This effectively increases device capacitance at a fixed operating bias and reduces photodetector bandwidth (RC-limit).
A low annealing temperature at 500° C. would be able to restrict dopant diffusion. However, this is at the expense of higher dopant activation and limits device integration compatibility with backend CMOS processes which uses temperatures of up to 600-700° C.
There is thus a need to address the above drawbacks for existing photodetectors.